Circuit arrangement with power factor correction as well as a corresponding appliance

ABSTRACT

The circuit arrangement has a mains connection, a mains switch and a switched-mode power supply which contains a power factor coil for power factor correction. In this case, the mains switch has two switching contacts, one of which is arranged in a supply line between the mains connection and the switched-mode power supply, and in this way switches the phase or neutral conductor of the 50 Hz line network off and on. The connections of the second switching contact are located in a voltage supply for the driver circuit of the switched-mode power supply, and the second switching contact switches the switching transistor in the switched-mode power supply off when the circuit arrangement is switched off, by its control voltage being switched off directly or indirectly. The switching contact of a relay is arranged in parallel with the first switching contact of the mains switch, and the control coil of this relay is connected to an output voltage of the switched-mode power supply. Since the output voltages of the switched-mode power supply are each buffered by a capacitor, the relay is thus still open for a certain time after the arrangement has been switched off. The energy which is stored in the power factor coil is thus dissipated by the relay, by the coil drawing the current which is required to dissipate the energy from the mains via the relay.

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/EP02/13782, filed Dec. 5, 2002, which waspublished in accordance with PCT Article 21(2) on Jun. 26, 2003 inEnglish and which claims the benefit of German patent application No.10162048.9, filed Dec. 17, 2001.

The present invention is based on a circuit arrangement having a mainsconnection, a mains switch with two switching contacts, a switchingelement, a power factor coil for power factor correction, and aswitched-mode power supply and on an appliance having a correspondingcircuit arrangement.

Switched-mode power supplies produce a highly pulsed load on the mainspower supply system, and this leads to harmonic currents in the mainspower supply system. This load occurs in the region of the voltagemaxima of the sinusoidal mains voltage, at which maxima anenergy-storage capacitor in the switched-mode power supply is recharged.Appliances having a relatively high power consumption, such astelevision sets with relatively large cathode ray tubes, must thereforenow comply with specific regulations with regard to harmonic currents.The harmonic load on the mains power supply system caused by anappliance can in this case be indicated by a so-called power factor.

Widely differing circuit concepts are known for improving the powerfactor, for example those disclosed in DE-A-196 10 762, EP-A-0 700 145and U.S. Pat. No. 5,986,898. These contain a second current path with acoil between the mains rectifier and the primary winding of thetransformer, with the inductance of this coil acting like a currentpump, which is controlled by the switching transistor and in this waybroadens the pulsed current flow in the switched-mode power supply.

A further possible way to improve the power factor of a switched-modepower supply is to use a coil in the input area of the switched-modepower supply. This coil is also called a 50 Hz coil, mains frequencycoil or power factor coil. In order to avoid confusion with other coils,the description therefore always uses the term power factor coil forthis coil. However, this power factor coil has the disadvantage that,when the mains switch is operated in order to switch off the appliance,the current flow in the power factor coil is interrupted suddenly. Theenergy stored in the coil must, however, be dissipated. Since the openmeans switch represents the highest impedance of the circuit, a veryhigh voltage is therefore developed across the switching contacts of themains switch, and this causes an arc. This means that the mains switchages more quickly, and that the switch represents a safety risk since,in the worst case, it is a potential source of fire.

Other mains switches in which the rate of opening of the switchingcontacts is low are likewise at risk. In this case, although the voltageacross the contacts is not very high, an arc is nevertheless producedand burns until the end of the corresponding mains half-cycle. This alsoresults in a large amount of energy being lost in the switch, whichleads to rapid ageing.

The object of the present invention is to specify a circuit arrangementas well as a corresponding appliance of the type mentioned initially,which have high reliability with circuit complexity that is as economicas possible.

The circuit arrangement according to the invention has a mainsconnection, a mains switch and a switched-mode power supply, whichcontains a power factor coil for power factor correction. The mainsswitch in this case has two switching contacts, one of which is arrangedin a supply line between the mains connection and the switched-modepower supply and the switched-mode power supply, and in this wayswitches the phase or neutral conductor of the 50 Hz line network offand on. The second switching contact is arranged in the voltage supplyfor the driver circuit of the switched-mode power supply and switchesthe switching transistor in the switched-mode power supply off when thecircuit arrangement is switched off, by switching off a control orsupply voltage.

The switching contact of a relay is arranged in parallel with the firstswitching contact of the mains switch, the control coil of which relayis connected to an output voltage of the switched-mode power supply.Since the output voltages of the switched-mode power supply are eachbuffered by a capacitor, the relay is thus still closed for a certaintime after the arrangement has been switched off. The energy stored inthe power factor coil is thus dissipated via the relay by the coildrawing the current which is required to dissipate the energy from themains via the relay. The relay does not open until the output voltagewhich is present across the relay has fallen, so that the current flowfrom the mains connection is interrupted completely.

The invention will be explained in more detail in the following text byway of example using an exemplary embodiment which is illustratedschematically in the figures, in which:

FIG. 1 shows an arrangement with a mains switch, a relay and aswitched-mode power supply.

FIG. 2 shows an arrangement with a relay, which has an additionalswitching contact for a demagnetization coil, and

FIG. 3 shows a current and voltage diagram of the circuit arrangementsat the time of switching off.

FIG. 1 shows, schematically, a switched-mode power supply which has arectifier means, in this example a bridge rectifier BR with four diodes,an energy-storage capacitor C1 and a transformer TR with a primarywinding W1, an auxiliary winding W2 arranged on the primary side as wellas secondary windings W3, W4 and W5 arranged on the secondary side. Theenergy-storage capacitor C1 is in this case arranged between the bridgerectifier BR and the primary winding W1. A switching transistor T1,which is controlled by a control voltage DS from a driver circuit DC, isconnected in series with the primary winding W1. A supply voltage VCC,for operating the switched-mode power supply is supplied to the drivercircuit DC, and is produced via the auxiliary winding W2, a diode D1 anda capacitor C2.

The switched-mode power supply in FIG. 1 is a part of a circuitarrangement which, in addition to a mains connection NA and a mainsswitch S1, has two switching contacts 1, 2. The circuit arrangement is,for example, integrated in a television set.

In order to control the switched-mode power supply, the driver circuitDC is supplied with a control signal RS, which is derived from a supplyvoltage U4 on the secondary side, for example the system voltage in atelevision set, and is transmitted via an optocoupler or an isolatingtransformer, which is not illustrated, to the primary side of theswitched-mode power supply. The so-called snubber network SN is arrangedin parallel with the primary winding W1 and is used to damp voltagespikes which occur when the switching transistor T1 is switched off.

The switched-mode power supply also contains a starting circuit AS,which supplies the driver stage DC (after the appliance in which thecircuit arrangement is integrated has been switched on) with power forthe starting phase of the switched-mode power supply. The startingcircuit AS is normally a high-impedance resistor chain, which produces aconnection between the bridge rectifier BR and the capacitor C2 in orderto produce the supply voltage VCC. During operation, the supply voltageVCC is then produced by the auxiliary winding W2, as well as the diodeD1 and the filter capacitor C2.

The switched-mode power supply as shown in FIG. 1 preferably operates onthe flyback converter principle, although other circuit principles arelikewise possible. Flyback converters are preferably used in appliancesfor entertainment electronics, for example in television sets and videorecorders. In this case, when the switching transistor T1 in a flybackconverter is switched on, energy is stored in the transformer TR and isthen transmitted, in the phase when the switching transistor is switchedoff, to the secondary windings W3-W5, and to the primary auxiliarywinding W2. Flyback converters are used both as AC/DC converters and asDC/DC converters.

Switched-mode power supplies of this type have a low power factor, sincethe energy-storage capacitor C1 is recharged only in the area of thevoltage maxima and minima of the 50 Hz mains voltage, when the outputvoltage from the bridge rectifier BR is greater than the voltage valueacross the energy-storage capacitor C1. One simple possibility forimproving the power factor of a switched-mode power supply is to use amains frequency coil or power factor coil NS, which is connected betweenthe mains connection NA and the energy-storage capacitor C1. In thisexemplary embodiment, it is connected between the mains switch S1 andthe mains rectifier BR.

This coil broadens and shifts the phase of the pulsed current flow whichis used to recharge the energy-storage capacitor C1, since theinductance of the power factor coil NS, for example 50 mH, causes thecurrent flow through the coil to rise only gradually, and to fall onceagain in a damped manner. Appropriate choice of the inductance value inthis case achieves a power factor that complies with the requirements.

Since the power factor coil NS is arranged in the current path of themains switch S1, the relatively high inductance of the power factor coilNS causes a high voltage when the switching contact 1 is opened, andthis leads to an arc in the switching contact 1. The mains switch S1 hastwo switching contacts 1 and 2, of which, according to the invention, afirst switching contact 1 is connected in a supply to the mainsconnection NA, and the second switching contact 2 is coupled to a supplyor control voltage for the driver circuit DC. The second connection b ofthe mains connection NA is connected to the bridge rectifier BR withoutbeing switched.

The second switching contact 2 is in this way used for switching off theswitching transistor T1, by using the driver circuit to switch off theswitching voltage DS for the switching transistor T1 directly orindirectly. The switching contact 2 may, for example, be connectedbetween the connecting points c′ and d′, so that the driver circuit DCis disconnected from the supply voltage VCC during the switching-offprocess. The switching transistor T1 is then switched off completelyafter only a few switching cycles.

A capacitor (not shown) is advantageously also connected to earthdownstream from the connection c′ and is used to prevent the switchingcontact 2 of the switch from bouncing, and also to provide filtering forthe long supply line to the mains switch S1. In this case, thecapacitors of the capacitor also influences the number of switchingcycles after which the switching transistor T1 is switched offcompletely.

However, another voltage, for example a control voltage for the drivercircuit DC can also be switched off in a corresponding manner by theswitching contact 2, or the switching contact 2 can be used to ensurethat the control signal RS is at a predetermined voltage value, so thatthe switching transistor T1 is likewise switched off permanently.

A bypass is arranged in parallel with the switching contact 1 of themains switch S1, and bridges this switching contact. This isadvantageously a relay R1 with mains isolation, although other switchingelements, for example a switching transistor, may also be used. When arelay R1 is used for mains isolation, an output voltage U2 on thesecondary side can be applied directly to the control coil ST of therelay 1, at the connection e. The mains isolation is in this caseindicated by a line N in FIG. 1.

When the mains switch S1 is opened, the switching contact 2 thusswitches off the switching transistor T1 directly or indirectly in ashort time, so that no more energy is transferred from the transformerTR to the secondary windings W2-W5. However, a current can still flowthrough the bypass, the switching contact 3 of the relay R1, so that themagnetic field in the power factor coil NS can be dissipated by means ofa current flow via the relay R1 without any arc being formed across theswitching contact 1 of the mains switch S1. This considerably improvesthe life of the mains switch S1.

The output voltage U2 is, for example, a rectified and smoothed voltage,which is obtained from the supply voltage U3 by means of a diode and arelatively large filter capacitor (which is not illustrated). Theswitching contact 3 of the relay R1 thus opens with a delay incomparison to the switching contacts 1 and 2, since the larger capacitordischarges with a considerable delay with respect to a switching cycleof the switching transistor. The time constant is in this case dependenton the capacitance of this capacitor and on the burden produced by theload. The connection a of the mains connection NA is not completelydisconnected from the switched-mode power supply until after this.

When the appliance is switched on by pressing the mains switch S1, theswitching contacts 1 and 2 are closed, so that the switched-mode powersupply can start via the switching contact 1 since, at the same time,the driver circuit DC is once again ready to operate via the switchingcontact 2. Once the switched-mode power supply has started up, then theswitching contact 3 of the relay R1 is also closed again by the outputvoltage U2, so that the appliance can then be switched off again withoutany arc in the mains switch S1. During switching on, the switched-modepower supply thus behaves in precisely the same way as when using themains switch S1 in appliances of a conventional type.

The appliance in which the switched-mode power supply is arranged has anormal mode and a standby mode, also referred to as a readiness mode,then it is advantageous to use as the voltage U2 an output voltage whichis produced on the secondary side and is switched off in the standbymode. The relay is then switched off in the standby mode, and does notconsume any energy. The switching contact 3 is then opened. However,there is no disadvantage in switching off the appliance when it is inthe standby mode since, in the standby mode, the power consumption ofthe appliance is so low that no arc can be formed across the switchingcontact 1 of the mains switch.

FIG. 2 shows a mains switch S1 with two switching contacts 1 and 2,which are connected as described in FIG. 1. However, a relay R2 is usedas the relay here, having a second switching contact 4, in parallel withthe switching contact 3. The switching contact 4 is in this caseadvantageously used for driving a demagnetization coil (which is notillustrated), which is normally used in television sets with a cathoderay tube or in corresponding computer monitors. The drive for thecontrol coil ST of the relay R2 in this case corresponds to the drivefor the relay R1 in FIG. 1. The relay R2 likewise has mains isolation,indicated by the line N. The same reference symbols are in this caseused for further connections and elements in FIG. 2 which correspond tothe equivalent connections and elements in FIG. 1.

Cathode ray tubes, which are used in television sets or computermonitors, require demagnetization from time to time, in order tomaintain the colour purity of the cathode ray tube. This is achieved bymeans of a demagnetization coil, to which an AC voltage is normallyapplied during the process of switching on the appliance. The 220 voltmeans voltage is used as the AC voltage in this case and produces alarge current surge at the time of switching on, which then graduallydecays. The decay is produced by means of a so-called posistor PS, whichis heated by the high current, with its impedance becoming high in theprocess.

Once the current surge has decayed, the posistor PS subsequentlyconsumes about 1 watt of power, however, since it is permanently heated.This is not good for the standby mode of an appliance, since the standbyconsumption, for example for television sets, is intended to be as lowas possible. A relay is thus frequently used to switch off thedemagnetization coil in the standby mode.

In one development of the invention, one connection of the posistor PSis now connected to the second switching contact 4 of the relay R2, anda second connection of the posistor PS is connected to the connection bof the mains connection NA. The demagnetization coil (which is notillustrated) is connected to the connections i, j. A secondary voltageis used as the control voltage U2 and is switched off in the standbymode, so that the demagnetization coil is likewise switched off by theswitching contact 4 in the standby mode. This saves the relay forswitching off the demagnetization coil, and the corresponding drive forthe relay.

The voltage U2 may therefore be present only in the normal mode, and itmust be available before the start of the deflection process in thecathode ray tube in order to avoid image disturbances caused by thedemagnetization process. However, an appropriate voltage is alreadyavailable in a television set, so that the relay R2 therefore not onlymakes it possible to avoid wear resulting from arc effects in the mainsswitch, but also means that the demagnetization coil is switched off inthe standby mode.

The operation of the circuit as illustrated in FIG. 1 will now beexplained in more detail using the current and voltage diagrams shown inFIG. 3. I1 is in this case the current flying through the switchingcontact 1 of the mains switch S1, and I2 is the current flowing throughthe switching contact 3 of the relay R1. U1 is the voltage applied tothe connection c′ of the driver circuit DC and U2 is the secondaryvoltage applied to the connections e and f.

Until the time t1, the switched-mode power supply is operated in thenormal mode. This clearly shows the 50 Hz current pulses in the currentI1 and I2 when the energy-storage capacitor C1 is recharged, damped bythe power factor correction coil NS. The currents I1 and I2 are in thiscase of equal magnitude, since the current is shared by the mains switchS1 and the relay R1.

The mains switch S1 is now operated, and the appliance is thus switchedoff, at the time t1. The voltage U1, the supply voltage for the drivercircuit DC in consequence falls immediately. The current flow I1 throughthe switching contact 1 is likewise interrupted immediately. Twice thecurrent I2 now flows through the switching contact 3 of the relay R1,since the capacitor C1 is once again completely recharged. As theprocess continues, the current I2 decays, however, by which means themagnetic field in the correction coil NS is also dissipated.

However, the voltage U2 falls only gradually after the time t1, sincethe filter capacitor for the secondary voltage U2 is discharged onlygradually. The contact 3 of the relay R1 does not open until the timet2, when the voltage U2 has fallen below a specific threshold value, sothat the connection a of the mains connection NA is completely isolatedfrom the mains after this time. The time t2 in this case occursapproximately 100 milliseconds after the time t1, so that no arc is nowproduced either across the switching contacts 1 or across the switchingcontacts 3 of the relay R1. No more current flow through the relay R1can be seen at the time t2.

Further refinements of the invention are within the capability of aperson skilled in the art. In particular, other suitable switchingmeans, such as transistors, may also be used as relays. A supply voltagewhich is produced by the high-voltage transformer in an appropriateappliance can also be used as the control voltage U2 for the relay R1 orR2. If a number of switched-mode power supplies are used in thearrangement, for example a first switched-mode power supply for thenormal mode and a second switched-mode power supply for the standbymode, then it is sufficient to switch the first switched-mode powersupply off using the switching contact 2, since the power consumption ofthe standby switched-mode power supply can be ignored. The invention islikewise not restricted to flyback converters, as already explainedabove, and can also be used for other switched-mode power supplyconcepts when power factor correction is required.

1. Circuit arrangement having a mains connection, a mains switchcomprising two switching contacts, a switching element, and aswitched-mode power supply, which has a transformer with a primarywinding, a rectifier means providing a rectified voltage for saidprimary winding, an energy-storage capacitor coupled with a terminal tosaid rectifier means and said primary winding, a switching transistorcoupled to said primary winding, a driver circuit for producing acontrol voltage for said switching transistor, and a power factor coilfor power factor correction coupled between said mains connection andsaid energy-storage capacitor, wherein a first of said switchingcontacts is arranged between said mains connection and said rectifiermeans, the second of said switching contacts is coupled to a supply orcontrol voltage for said driver circuit in order to switch off saidcontrol voltage for said switching transistor, and a switching contactof said switching element is arranged in parallel with said firstswitching contact of said mains switch for maintaining a current aftersaid circuit arrangement is switched off by means of said mains switch.2. Circuit arrangement according to claim 1, wherein said transformerhas an auxiliary winding for producing a supply voltage for said drivercircuit, and wherein said second switching contact is arranged betweensaid auxiliary winding and said driver circuit in order to switch offsaid supply voltage.
 3. Circuit arrangement according to claim 2,wherein a diode as well as a capacitor for producing said supply voltageare arranged at one connection of said auxiliary winding, and whereinsaid second switching contact is arranged between said capacitor andsaid driver circuit.
 4. Circuit arrangement according to claim 1,wherein said switching element is a relay.
 5. Circuit arrangementaccording to claim 4, wherein a rectified and filtered output voltagefrom said switched-mode power supply is applied to one connection of acontrol coil of said relay, such that said switching contact of saidrelay opens if the output voltage falls, and in that, when saidarrangement is switched off, the supply voltage or control voltage fallsmore quickly than the output voltage of said switched-mode power supply.6. Circuit arrangement according to claim 5, wherein said switched-modemode power supply has a normal mode and a standby mode, in that saidoutput voltage is produced by a winding arranged on the secondary side,and in that said output voltage is switched off in said standby mode. 7.Circuit arrangement according to claim 6, wherein said relay has asecond switching contact, which is arranged between said mainsconnection and a demagnetization coil so that said demagnetization coilis switched off in said standby mode.
 8. Circuit arrangement accordingto claim 1, wherein said power factor coil is arranged between saidfirst switching contact and said rectifier means.
 9. Appliance, whereinsaid appliance has a circuit arrangement having a mains connection, aswitching element, a mains switch which has two switching contacts, anda switched-mode power supply, which has a transformer with a primarywinding, a rectifier means providing a rectified voltage for saidprimary winding, an energy-storage capacitor coupled to said rectifiermeans and said primary winding, a switching transistor coupled to saidprimary winding, a driver circuit for producing a control voltage forsaid switching transistor, and a first of said switching contacts isarranged between said mains connection and said rectifier means, a powerfactor coil for power factor correction coupled between said mainsconnection and said energy-storage capacitor, wherein the second of saidswitching contacts is coupled to a supply or control voltage for saiddriver circuit in order to switch off said control voltage for saidswitching transistor, and a switching contact of said switching elementis arranged in parallel with said first switching contact of said mainsswitch for maintaining a current after said circuit arrangement isswitched off by means of said mains switch.
 10. Appliance according toclaim 9, wherein said appliance has a cathode ray tube with ademagnetization coil.